Endocardial pacing leads having fixation structures at or near the distal tip are well known in the art and widely used. Some leads employ passive fixation or anchor structures, such as in the use of tines. However, it is also popular to use an active anchoring element, such as a retractable helical screw-in element, which is activated by the physician. Likewise, even tines can be designed to be retractable, and changed from a retracted state where they lie essentially along the circumference of the lead to an active or extended state. The activation or movement of such an anchor element can be achieved by applying a rotating, or torsional force to the coil element that runs the length of the lead, the rotational movement being transmitted the length of the lead to rotate the anchor element in one direction or another. Thus, the user rotates the connector pin on the proximal end of the lead, which pin in turn is connected to the coil, the coil being connected to the helix or anchor mechanism at the distal end of the lead.
In practice, the conductor coil through which the torsion is transmitted the full length of the lead has rather bad torsion characteristics. This is because the inner coil is generally designed to be flexible, to meet other requirements. Also, friction between the inner wall of the lead casing and the coil can be very high, especially when the tubing, or tubing, is made of a material such as silicone rubber. The result of such high friction is that torque transmitted to the coil is in turn carried also by the casing, such that a lot of the torsion is taken up in twisting the entire lead instead of being transmitted directly through to the distal end. In practice, the user may have a very uncontrolled feeling, and may be required to rotate the connector pin eight times or more in order to rotate the anchor mechanism one time.
In order to achieve good torsion, the ideal situation would be that the lead coil has no flexibility, such that it transmits all of the torsion (torque) inputted at the proximal end through to the element at the distal end. Also, in order to optimize transmission of torsion there would ideally be no friction between the coil and the inner wall of the tubing, so that none of the rotation is transmitted through to the tubing. Of course, the problem that has always faced the art area is that the coil needs to be flexible for other purposes. Also, since it is desirable to minimize the diameter of the lead, it is difficult to reduce friction between the coil and the inner wall of the outer casing.
Heretofore, there has not been a solution in the pacer lead area to the problem of providing a lead with the required flexibility, but yet optimizing the ability of the lead to transmit torque through to the distal end. Leads are known which have different stiffening characteristics along their lengths. Thus, in U.S. Pat. No. 4,493,329, Crawford et al., stiffening is provided to the J portion of an atrial lead. Also, in U.S. Pat. No. 4,135,518, Dutcher, assigned to Medtronic, Inc., a section adjacent to the distal electrode is provided with a greater flexibility, while the remainder of the lead is relatively stiffer. The purpose of this second design is to provide a first segment of the lead which is capable of bearing an axial mechanical force, and a second segment which is more flexible and incapable of sustaining axial mechanical force. However, this lead design does not provide the type of improved torsion characteristics as are desired to overcome the prior art problems as set forth above.